1. Field of the Invention
The present invention relates to a pneumatic tire comprising a shoulder rib segmented into a main rib located at the inner side thereof in a width direction of the tire and a thin rib locate at the outer side thereof in the width direction of the tire by a thin groove extending along a circumferential direction of the tire.
2. Description of the Related Art
Pneumatic tires with a rib-based tread pattern for long-distance trucks or buses have the following problem. That is, the tires tend to suffer irregular wear in which shoulder rib thereof wears away earlier than a central area of the tread. Therefore, even when the tire has not been worn so heavily yet as a whole, the tire needs to be replaced with a new one due to the irregular wear.
As preventive measures against the above problem, for example, Japanese Unexamined Patent Publication No. 2004-268783 and Japanese Published Patent Publication No. 2002-512575 disclose tires such that a low rigidity thin rib is formed on a shoulder rib, which is segmented by a thin groove extending in a circumferential direction of the tire, to thereby prevent the irregular wear.
In the above-mentioned tires, when the tire is deformed, the thin rib is tilted inwardly in a width direction of the tire and received by a main rib. When the thin groove is arranged to have a large width, the thin rib deflects a large amount, and an edge portion of the thin rib comes into contact with an inner wall surface of the main rib and the inner wall surface thereof is closer to the bottom of the groove and lower than an edge portion of the main rib. As a result, when the tire is deformed, since the edge portion of the main rib is exposed, the protective performance for the edge portion of the main rib is reduced, and the edge portion of the main rib suffers damage. Such damage on the edge portion of the main rib functions as an inception and may develop, to an irregular wear on the entire shoulder rib around the damage as a core. Therefore, when the width of the thin groove is too large, preventive effect against the irregular wear may be reduced.
On the other hand, when the thin groove is formed in a small width and when the tire is deformed, only a small deflection is generated on the thin rib. As a result, when the thin rib is tilted inwardly in a width direction of the tire, an edge portion of the thin rib comes close to the edge portion of the main rib. As result, the protective performance on the edge portion of the main rib is increased. Since the edge portion of the main rib is prevented from being damaged, the preventive effect against the irregular wear of the shoulder rib is increased. However, when the width of the thin groove is reduced, a thin groove forming blade on a tire-producing die also needs to be thin enough. As a result, the durability of the blade is reduced, and the blade may be bent or broken while removing the tire from the die. Thus, the prevention of the irregular wear of the shoulder rib and the durability of the thin groove forming blade are in an antinomic relationship.